1 Technical Field
The present invention relates to methods of surface treating substrates and to methods of manufacturing (Group) III-V compound semiconductors; in particular the invention relates to substrate surface treating and III-V compound semiconductor manufacturing methods that enable the reduction of post-epitaxial-growth surface roughness.
2 Description of the Related Art
Substrates composed of Group III-V semiconductor compounds have been widely used for semiconductor lasers, LEDs, and high-speed devices because they have luminescent properties and high electron mobility. Surface treatment is typically carried out in methods of manufacturing substrates composed of Group III-V semiconductor compounds. Such surface treatments include, for example, the surface treatment method disclosed in Japanese Unexamined Pat. App. Pub. No. 2001-189278, and the method of cleaning an InP wafer disclosed in Japanese Unexamined Pat. App. Pub. No. S62-252140.
In the surface treatment method disclosed in Japanese Unexamined Pat. App. Pub. No. 2001-189278, the front side of GaAs as a compound semiconductor is surface treated with ozonated water prior to epitaxial growth in order to reduce Si or silicon compounds on the surface.
In the InP wafer cleaning method disclosed in Japanese Unexamined Pat. App. Pub. No. S62-252140, the surface of InP is treated with a liquid mixture of phosphoric acid-hydrogen peroxide-water or a liquid mixture of hydrogen fluoride-hydrogen peroxide-water.
However, if the typical surface treatment method by an ozonated water process disclosed in Pat. App. Pub. No. 2001-189278 is applied to InP, the surface roughness of InP does not decrease. The reason is as follows. Ozonated water typically exhibits neutrality. If the surface treatment is performed with neutral ozonated water, the surface becomes Group III-rich because the stoichiometry (stoichiometric composition) of the surface of Group III-V is not optimum. As a result, in epitaxial growth the surface is likely to become roughened because Group III oxide hinders epitaxial growth.
In the surface treatment of the front side of GaAs with ozonated water disclosed in Japanese Unexamined Pat. App. Pub. No. 2001-189278, microscopic roughness (haze) in the surface after epitaxial growth is larger than before epitaxial growth. The reason is as follows. Ga generates a dense oxide because Ga has a tendency to oxidize readily with ozonated water. With the generation of a dense oxide on the Ga surface, it is likely to become roughened when an epitaxial film is grown on the surface, because the oxide hinders the epitaxial growth.
In the method of cleaning the surface of InP wafer with a liquid mixture of phosphoric acid-hydrogen peroxide-water or a liquid mixture of hydrogen fluoride-hydrogen peroxide-water disclosed in Japanese Unexamined Pat. App. Pub. No. S62-252140, microscopic roughness of the surface increases following epitaxial growth. The reason is as follows. The acid based treatment liquid used in Japanese Unexamined Pat. App. Pub. No. S62-252140 makes the acid concentration too high (the pH too low). Consequently, dense Group V oxide is generated on the surface and hinders epitaxial growth. This causes haze on the InP surface following epitaxial growth to increase. The overly high concentration of the aqueous hydrogen peroxide leads to intense localized reactions arising due to the unevenness of the oxide on the surface or to foreign matter that clings to the surface, as a consequence of which minute surface irregularities can arise. Meanwhile, neutral or alkaline treatment solutions cannot eliminate Group III oxide generated on the original surface, such that remaining Group III oxide hinders epitaxial growth, and microscopic roughness on the surface further increases. The microscopic surface roughness hinders electrons from moving through the high-purity channel layer in a high-speed device such as an HEMT and consequently compromises the device's electrical properties.